Piezoelectric device



Jan. 22, 1946. Q RT 2,393,429

PIEZOELECTRIC DEVICE Filed March 22, 1943 CURRENT a TIMER CONTROL MEA Sf5 /7 POTENTIAL I /3 souncs Patented Jan. 1946 rmzouteo'rarc DEVICEFrank smnargM eifia, Oliibrassignor uni? Brush Development Company,

Cleveland,

Ohio, a corporation of Ohio Application March 22, 1043, Serial No.480,113

. 26 Claims. (01. 121-321) This invention relates primarily topiezoelectric devices and, more particularly to methods of manufacturingtransducer elements of the type constituted by two or more crystalsections amxed to each other for mutual constraint and provided withsuitable electrodes. The invention also relatesto the joining togetherof fusible or thermoplastic non-conductive materials of all kinds, such,for example, as cellulose acetate, Vinylite, or the like.

If information is desired with respect to the theory underlying theoperation of transducer units of the types to which this inventionprimarily pertains, attention maybe directed to the United Statesreissue patents to C. B. Sawyer,l Re. 20,213, and Re. 20,680. Whenconstituted by two sections cemented together for mutual constraint,units of the type shown in the Sawyer patents are generally known to thepublic .under the registered trade-mark Bimrph," and are also calledmultiplate flexing elements." Previous to the cementing together of thesections constituting a multiplate flexing element, the major facesthereof are usually'provided with conductive electrodes of colloidalgraphite in accordance with the method disclosed in the United Statespatent to Alfred L. W. Williams, No. 2,106,- 143, to which appropriatelead extensions are afilxed.

Multiplate flexing elements may also be falbricated in accordance withthe teaching of the United States patent to J. H. Ream, No. 2,266,- 333;when so made they are considered to have advantages not attainable bythe Williams method although theelectroded surfaces of the individualsections are cemented to each other in the usual manner.

The step of cementing the crystal sections together obviously involves adefinite labor and material cost. Furthermore, no cement giving entiresatisfaction has been discovered although many kinds, includingpre-melted crystalline material itself, have been tried.

It is an object of this invention, therefore, to provide an improvedpiezoelectric unit that shall be devoid of cement :between the opposedsurfaces of the constituent crystal sections.

Another object of the invention is to provide a method of fabricatingpiezoelectric units of the type described.

Ancther object of the invention is to provide a method of fabricatingpiezoelectric units of the type described, whereby the individual costthereof shall be lessened.

of joining together structural elements or sections of a fusible,electrically non-conducting material such as cellulose acetate,Vinylite, "Styron, and the like.

For convenience in describing and claiming the invention, Rochelle saltand piezoelectric crystalline mixtures isomorphic therewith will bedesignated R-type" material; primary phosphates, primary arsenates andpiezoelectric crystalline mixtures isomorphic therewith will bedesignated "P-type material. Inasmuch as the invention has achievedmarked success in connection with Rochelle salt and with primaryammonium phosphate those compounds will be referred to particularlyhereinafter in disclosing the invention, but the reference thereto isnot to be construed as a limitation because the invention is applicableequally as well to all other fusible piezoelectric materials.

The term section, as employed hereinafter, is to be construed as meaningeither a plate cut from a crystal of fusible piezoelectric material oras defining a structural element of any fusible or thermoplasticmaterial that is substantially a non-conductor of electricity, such, forexample, as the materials enumerated above.

The improved method, as practiced in the fabrication of a multiplateflexing element from A still further object is to provide a method twoR-type crystal sections, or from two sections of P-type material, havingplanar faces, pref-- erably comprises four steps, namely:

1. Depositing a colloidal graphite electrode on one face of one sectionor on one face of each section.

2. Positioning the said planar faces in contact with each other so thatthe graphitic material is disposed between them, and providingconductive lead-ins at opposite ends of the said material.

BLPressing said electroded surfaces together while passing an electriccurrent through the graphite layer by way of the lead extensions, theduration of current-flow and the amplitude of the current being justsuiilcient to cause melting of the crystal sections adjacent to theelectroded surfaces. 1

4. Discontinuing the application of potential and thereafter maintainingpressure on the sec tions until they have cooled.

The novel features considered characteristic of the invention are setforth with particularity in the appended claims. The method itself,however, together with further objects and advantages of the resultingproduct, will best be understQQ from a consideration of the followingdescription thereof, when read in connection with the accompanyingdrawing, in which:

Figure 1 is a view in perspective of a crystal section, having acolloidal graphite electrode to the opposite ends of which a leadextension has been applied;

Figure 2 is a view in perspective of a partially completed piezoelectricunit that has been constructed according to this invention, and

Figure 3 is a diagrammatic, conventionalized view exemplifying themethod of the invention.

In all figures of the drawing, identical elements are similarlydesignated.

Referring now to Figures 1 and 2, in fabricating a multiplate flexingelement according to the invention one face of a section i ofpiezoelectric material, either R-type, such as Rochelle salt, or P-type,such as primary ammonium phosphate, is provided with a restrictedsurface coating 3 of colloidal graphite or the like, preferably byfollowing the method outlined in the Williams patent. A similarlydimensioned section 5 of crystalline material is then selected, whichmay or may not be provided with a similar graphitic coating, dependingupon whether a high or a low electrode resistance is desired in thefinished product.

After thoroughly drying the coating or coatings the sections are placedtogether in face-to-face relation, with the surfaces to be joined incontact with each other and in registry, to form a stack as shown inFigure 2. At the time the stack is formed the unit may be provided withtwo lead extensions '1 of metallic foil, the inner ends of which,respectively, are disposed in conductive contact with the opposite endsof the graphitic material. Alternatively, as illustrated in Figure 1,the lead extensions may be affixed to one of the electroded sections inadvance of forming the stack. If cement of any sort is utilized forholding the extensions in place it should be confined to the uncoatedmargin of the crystal face and not be interposed between the extensionand the graphite layer.

Referring now to Figure 3, after the stack has been formed, it is placedupon a smooth supporting surface 9, preferably heat conductive, and theelectroded faces of the sections are urged toward each other byappropriate means, such, for example, as a metallic weight ll.Thereafter, the lead extensions are connected to a suitable source ofpotential I3, alternating or unidirectional, and electric current iscaused to flow through the graphitic material for the purpose ofgenerating heat sufiicient to fuse the crystal substance directlyadjacent thereto. An automatic timer l5 and a potential regulatingdevice ll, such as a rheostat or the like, may be interposed in theheating circuit for s0 limiting the current and the duration thereofthat damage is not done to the sections.

It is substantially impossible to give specific values of currentnecessary for satisfactory operation, or to state the length of timethat the current should flow in all instances. These factors can bedetermined only by experiment, inasmuch as they depend upon theresistance of the graphite layer, the dimensions of the crystal sectionsand the particular piezoelectric material employed.

It may be stated in general, however, that it is preferable to maintaina relatively high temperature over as short a time interval as possible,directly at the electroded surfaces, rather than to heat the electrodesto a lower temperature over a longer time period. The cooling of theunit, after the welding operation is finished, also should be carefullycontrolled, if the sections are thin, in order to avoid cracking.

Merely by way of example, in manufacturing a muitiplate flexing elementapproximately one inch square, the graphite layer had a thickness ofapproximately .0005, and was substantially coextensive with the sectionfaces in contact with each other. The resistance of the layer, from endto end, before the heating current was caused to flow therethrough, wasof the order of 2,000 ohms and it required 45 milliamperes for severalseconds to produce a satisfactory joint.

As a result of the heat and pressure, the fused layers of the sectionscoalesce, the graphitic material is permanently bonded to the leadextensions, and contact between the graphite and the crystal mass per seis improved. Also, quite contrary to expectation, the resistance of thegraphite electrode may be kept at a satisfactory value although it isprobable that the coalescing of the fused material interrupts thecontinuity of the layer to some extent.

Insofar as the character of the bond between the cooled sections isconcerned, it is a matter of interest to note that the weld actually isstronger than the remainder of the crystalline mass and that it resistsshearing stresses many times greater than the cements, heretoforeemployed, are capable of withstanding.

Instead of passing current directly through the graphite by conductivelyconnecting it to a source of potential, it may be heated inductively bymomentarily exposing the unit to an alternating electromagnetic fieldhaving the requisite frequency and density. Such alternative, beingreasonably obvious in view of Gravley Serial Number 576,561, ArndtSerial Number 471,968, and Arndt Patent Number 2,388,242, has not beenillustrated. Also, it is believed obvious that granular materials otherthan graphite may be utilized, if desired, and that the material may bedry or wet.

Outer electrodes may be applied to the unit either by following themethod described in the Williams patent, the method disclosed by Ream orby any other well-known method and the unit, thereafter, may bewaterproofed as is customary.

From a consideration of the foregoing, it will be evident that the newmethod of joining crystal sections and of providing the unit with acentral electrode is a decided improvement over methods heretoforeknown. Not only does it enable a substantial reduction to be made in thecost of labor and material, but the units produced thereby are veryuniform and are appreciably more eflicient and durable than thoseproduced by previously known methods, such as those referred tohereinbefore.

As pointed out earlier in this specification the new method is notlimited to the fabrication of crystalline multiplate flexing elementsbut it may also be employed for joining together plates or otherstructural elements, large or small, of other fusible materials that aresubstantially non-conducting. It thus lends itsr f, for example, to theproduction of laminated sheets of cellulose acetate if transparency isnot required, or to the joining together of thermoplastic materials forwhich no satisfactory cement is available.

The inventor is fully aware of the fact that numerous modifications ofthe disclosed method will be apparent to those skilled in the art towhich the invention pertains. The invention, therefore, is not to belimited except insofar as is necessitated by the prior art the appendedclaims.

What is claimed is:

1. The steps in a method of electroding the surface of a section offusible piezoelectric matter, that comprise positioning on said surfacea layer of granular electrically conductive material, heating said layerto a temperature sufficiently high to cause incipient fusion of thepiezoelectric matter immediately adjacent thereto and discontinuing theheating before damage is done to the remainder of the section, wherebysaid layer is intimately bonded to the section to constitute anelectrode therefor.

2. The method as set forth in claim 1, characterized in that thepiezoelectric matter is taken from the group hereinbefore designatedR-type.

3. The method as set forth in claim 1, characterized in that thepiezoelectric matter is taken from the group hereinbefore designatedP-type.

and by the spirit of 4. The method as set forth in claim 1,characterized in that the piezoelectric matter is Rochelle salt. 1

5. The method as set forth in claim 1, characterized in that thepiezoelectric matter is a primary phosphate.

6. The steps in a method of joining together two structural elements offusible material, that comprise providing a surface of at least one ofsaid elements with a coating of granular electrically conductingmaterial, bringing a surface of the other element in forcible contactwith the coated surface, passing an electric current through saidcoating at such amperage and for such time as will cause fusion of thefusible material immediately adjacent to the coating, discontinuing theflow of current, maintaining the forcible contact until the fusedmaterial has substantially solidified, and thereafter discontinuing theapplication of force to the joined elements.

'7. The method as set forth in claim 6. characterized in that thespecific electrical resistance of the structural material is high withrespect to the specific electrical resistance of the conductingmaterial.

8. The steps in a method of fabricating transducer units from at leasttwo sections of crystalline piezoelectric matter each of which has anextended substantially planar surface, that comprise positioning on atleast one of said surfaces a layer of granular electrically conductingmatter, urging said surfaces toward each other to exert pressure uponsaid layer, causing an electric current to flow through said layer atsuch amperage and for such period oftime as will occasion the heatingthereof to a degree sufficient to cause fusion of the piezoelectricmatter closely adjacent thereto, stopping the flow of current beforefusion has extended to the remainder of each section, and thereafterpermitting the unit to cool while maintaining the pressure.

9. The method as set forth in claim 8, characterized in thatthepiezoelectric matter is taken from the group hereinbefore designatedR-type.

10. The method as set forth in claim 8, characterized in that thepiezoelectric matter is taken from the group hereinbefore designatedP-type.

11. The method as set forth in claim 8, characterized in that thepiezoelectric matter is Rochelle salt.

12. The method as set forth in claim 8, characterized in that thepiezoelectric matter is a primary phosphate.

13. The method as set forth in claim 8, characterized in that thepiezoelectric matter is primary ammonium phosphate.

14. The method as set forth in claim 8, characterized in that thegranular material is graphite.

15. A transducer assembly comprising a body of fusible piezoelectricmatter, an electrode constituted by -a layer of granular electricallyconductive material and an autogenous joint connecting said body ofpiezoelectric matter to said electrode.

16. A transducer assembly comprising at least two bodies of fusiblepiezoelectric matter, an au togenous joint connecting said two bodiestogether, and an electrode of granular electrically conductive materialsubstantially coaextensive in area with the joint and integrallyincluded therein.

17. A transducer assembly comprising at least two plates of fusiblepiezoelectric matter disposed in face-to-face relation, granularelectrically conductive material interposed between said faces, saidfaces being united to each other and to said conductive material througha solidified autogeneous-joint.

18. An assembly as defined in claim 17, characterized in that thepiezoelectric matter is taken from the group hereinbefore designatedR-type.

19. An assembly as defined in claim 1'7, characterized in that thepiezoelectric matter is taken from the group hereinbefore designatedP-type.

20. The steps in a method of joining together two structural elements offusible material, that compries providing a surface of at least one faceof at least one of said elements with a coating of granular electricallyconducting material extending over less than the entire area of saidface, bringing a surface of the other element in forcible contact withthe surface having said coating, passing an electric current throughsaid coating at such amperage and for such time as will cause fusion ofthe fusible material immediately adjacent to the coating including theuncoated fusible material to the side of said coating, discontinuing theflow of current, maintaining the forcible contact until the fusedmaterial has substantially solidified, and thereafter discontinuing theapplication of force to the joined elements.

21. The steps in a method of joining together two structural elements offusible piezoelectric material that comprises providing a surface of atleast one of said elements with a coating of granular electricallyconducting material, placing a lead of electrically conducting materialin engagement with said granular material at each of two spaced points,bringing a surface of the other element in forcible contact with saidleads and with the surface having said coating, passing an electriccurrent through said leads and through said coating at such amperage andfor such time as will cause fusion of the fusible material immediatelyadjacent to the coating, discontinuing the flow of current, maintainingthe forcible contact until the fused material has substantiallysolidified, and thereafter discontinuing the application of force to thejoined elements.

22. An assembly comprising a body of fusible piezoelectric matter, anelectrode constituted by a layer of granular electrically conductivematerial on a face of said body, said electrode layer extending overless than the entire area of said face, and an autogenous joint betweensaid body and said layer.

23. An assembly comprising two bodies of fusible piezoelectric matter,an electrode constituted by a layer of granular electrically conductivematerial on a face of at least one of said bodies, said electrode layerextending over less than the entire area of said face, and an autogenousJoint connecting said two bodies together with said layer of granularmaterial between said two bodies and integrally included in said joint,and said joint including the unelectroded area of said electroded face.

24. An assembly comprising two bodies of fusible piezoelectric matter,an electrode constituted by a layer of granular electrically conductivematerial on a face of at least one of said bodies, said electrode layerextending over less than the entire area of said face with theunclectroded area of said face defining a peripheral margin, anelectrically conductive lead extension in contact with said electrodelayer and extending over said unelectroded margin, and an autogenousjoint connecting said two bodies together with said electrode layerbetween said two bodies and integrally included in said joint and withone end of said conductive lead extension between said two bodies.

25. The method as set forth in claim 1, further characterized in this:that electric current is passed through said layer to cause heatingthereof by the resistance of the layer to the flow of current.

26. The steps in a method of joining together two bodies of fusiblematerial, that comprises providing a surface of at least one of saidbodies with a coating of comminuted material whose electricalconductivity is high compared to the electrical conductivity of said twobodies, bringing a surface of the other body in forcible contact withthe coated surface, inductively passing an electric current through saidcoating for such time as will cause fusion of the fusible materialimmediately adjacent to the coating, discontinuing the flow of current,maintaining the forcible contact until the fused material hassubstantially solidified, and thereafter discontinuing the applicationof force to the joined bodies.

FRANK SWINEHART.

